The copolymerization of ethylene and vinyl acetate is well-known; see Roedel, U.S. Pat. No. 2,703,794. In general, such copolymerization is effected by high pressure reaction of ethylene and vinyl acetate in an autoclave in the presence of a suitable initiator, such as ferrous sulfate, sodium bisulfite and water. Processes for the continuous polymerization of ethylene and vinyl acetate are also known; see U.S. Pat. No. 3,509,115 to French. The copolymers are also readily obtained by copolymerization of the monomers in the absence of water, e.g., in the presence of benzene; see Raetzsch and Musche, Plaste und Kautschuk, 21 (1), 16-18 (1974), Roedel, U.S. Pat. No. 2,519,791, and Hanford, U.S. Pat. No. 2,396,785, (Example 16). Furthermore, a comprehensive survey of ethylene and vinyl acetate copolymers is presented in "Ethylene and Its Industrial Derivatives", S. A. Miller, Edit., published by Ernest Benn, London, 1969, particularly pages 437-475, and in "Encyclopedia of Polymer Technology", Vol. 6, pages 387-420, published by John Wiley, New York, N. Y., 1967.
Copolymers of ethylene and vinyl acetate having a high ethylene content, e.g., greater than about 50 percent, particularly about 70-85 percent, are stiff and partially crystalline in the unmodified state. If the vinyl acetate content ranges between about 20 and 60 percent, the polymers are generally amorphous, somewhat flexible, and relatively transparent. These copolymers have found substantial commercial use, especially in the manufacture of water- and grease-resistant coatings for paper, modifiers for waxes, and in hot melt adhesives. The copolymers containing about 40-60 percent vinyl acetate exhibit rubbery properties and may be classified as elastomers. Although they do not contain a site for sulfur vulcanization, they can be cured with peroxides. These elastomers exhibit poor resistance to swelling when in contact with hydrocarbon oils, but good low temperature and mechanical properties. They also possess good oxidation resistance.
Peroxide curing, providing covalent cross-links between adjacent, substantially linear or branched chains, was demonstrated by Madge, Chem. and Ind. 1811 (1962). Peroxide curing is regarded by the rubber industry as an undesirable technique, as opposed to sulfur vulcanization. Operating hazards are often associated with the use of peroxides, they frequently have an objectionable odor and, more seriously, they are not readily adaptable to the broad formulation techniques developed for typical sulfur-based vulcanization systems. For these and other reasons, ethylene-vinyl acetate copolymers have found only limited use as elastomers despite the potentially attractive properties, mentioned above.
Kaizerman and Saxon, U.S. Pat. No. 3,972,857, describe modified ethylene-vinyl acetate copolymers containing an activated halogen atom cure-site for sulfur vulcanization. The polymers contain 35-80 percent ethylene and 0.2-4.5 percent, by weight, of chlorine or bromine, the remainder being vinyl acetate. The elastomers are made primarily by transesterification to introduce halogen-containing acetate groups into an ethylene-vinyl acetate copolymer, although it is also disclosed that they can be obtained by copolymerizing ethylene, vinyl acetate, and a halogen-containing vinyl monomer.